It is well known in the art that such sliding products (cements, limes, fillers, etc.) are normally contained in silos that are internally provided with one or more devices (such as one or more compressed air jets) for fluidizing with a gas the powdery product even before its entry into the trough of the screw conveyor.
Therefore, the term “aeration” refers here to the properties of certain materials composed of fine particles (such as, e.g. cements, limes, fillers, etc.) in whose mass the particles are separated by means of the absorption and the distribution of fluidizing air.
For those reasons, the density of the materials during use is reduced, and the particulate/gas mixture temporarily exhibits some of its fluid properties.
Usually, the greater the aeration, the greater the fluidity of the aerated mass.
Moreover, it is already known that the chance that such an aerated material has quasi-fluid behaviours is inversely proportional to the size of the particles forming the material mass.
In some embodiments, the blowing of fluidizing air into the material may go together with mechanical or pneumatic vibrations induced in the material mass to obtain a quasi-fluid behaviour.
Moreover, it is well known that, beside air, other gases may be used to give appreciable fluidity to the powdery material such as, e.g. nitrogen, carbon dioxide, etc. However, it has been found that the efficiency of the system increases by limiting as much as possible the turbulence within the quasi-fluid.
In this regard, it has been experimentally found that the turbulence is directly proportional to the discrete jumps of the conveyor screw pitch; in this sense, also the welding of single screw stretches represents discrete jumps with a consequent loss of efficiency of the conveyor device.
Therefore, there is a need for studies aiming to the making of conveyor screws (also known as “Archimedes screws”) designed to convey aerated powdery materials with an optimal power consumption, which is significantly lower than that of current devices.
Therefore, one of the objects of the present invention is the reduction of the power absorbed by the device when conveying the material through a series of structural approaches.
Normally, in prior art embodiments, the entire screw conveyor has the same pitch at any point.
However, a recent thorough study of mechanical and fluid-dynamic phenomena occurring within the screw conveyor has shown the importance of dividing the screw conveyor into screw portions with different functional characteristics with the aim of maximizing the efficiency of each screw portion.
Therefore, screw conveyors have been created that are divided into screw portions, arranged in series, with different geometric characteristics; each screw portion having a constant pitch other than the one of the previous screw portion and the one of the next screw portion.
In such newly conceived embodiments, the different screw portions have different pitches to perform particular functions such as extracting, compacting and conveying the material.
For example, EP-A2-0 816 938 (MITA INDUSTRIAL Co. LTD) discloses a solution involving the use of different screw portions with different pitches, arranged in series. However, each screw portion has the same pitch at any point.
However, in spite of some positive aspects, these solutions do not solve the problem of minimizing the power consumption to convey powdery material and maximizing the filling of the spires.
It has also been noted that if transfer screws made according to the teaching of EP-A2-0 816 938 (MITA INDUSTRIAL CO. LTD) are sloping at a certain angle (e.g. to lift the powdery material from the ground to the floor of a building) they show a low efficiency (power loss) in conveying the powdery material.
Moreover, U.S. Pat. No. 3,056,487 (KIPPER) discloses a transfer screw to convey a fibrous waste material deriving, e.g. from sugar cane processing.
The screw conveyor described in U.S. Pat. No. 3,056,487 (KIPPER) includes a transfer screw and a trough containing the transfer screw. The screw pitches vary according to a law of continuous variation.
However, the screw conveyor described in U.S. Pat. No. 3,056,487 (KIPPER) is not suitable for being installed in a complex plant to convey an aerated powdery material, e.g. a mixture of powdery cement and air (or any other suitable gas). In particular, this screw conveyor is not suitable because it does not allow an optimal filling of the spires of the transfer screw during the filling step, on the one hand, and during the accelerating and launching step, on the other.
Therefore, the technical solution proposed by the present invention intends to overcome the aforementioned disadvantages.